Molecular-targeted cancer therapeutics have shown impressive activity in the clinic. Some of the best noted examples include the tyrosine kinase inhibitors imatinib in Philadelphia chromosome-positive chronic myelogenous leukemia (CML) or KIT/PDGFR-mutant gastrointestinal stromal tumors (GISTs) and erlotinib in EGFR-mutant non-small cell lung cancer (NSCLC) (Krause, D. S. and R. A. Van Etten (2005) N. Engl. J. Med. 353(2):172-187). Treatment with these agents has led to dramatic anti-tumor responses in patient populations harboring these molecular abnormalities. However, despite the impressive initial clinical responses, most patients eventually progress due to the acquisition of drug resistance (Engelman, J. A. and J. Settleman (2008) Curr. Opin. Genet. Dev. 18(1):73-79). Identification of mechanisms of resistance have consequently opened the door to more rational drug combinations and the development of “second-generation” inhibitors that can potentially overcome or avoid the emergence of resistance.
Medulloblastoma is a primitive neuroectodermal tumor of the cerebellum that represents the most common brain malignancy in children (Polkinghorn, W. R. and N. J. Tarbell (2007) Nat. Clin. Pract. Oncol. 4(5):295-304). One form of treatment for medulloblastoma is adjuvant radiation therapy. Despite improvements in survival rates, adjuvant radiation is associated with debilitating side effects, thus supporting the need for new molecular targeted therapies.
The Hedgehog (Hh) signaling pathway has been directly implicated in the pathogenesis of medulloblastoma. Constitutive Hh signaling, most often due to underlying loss of function mutations in the inhibitory receptor PTCH1, has been demonstrated in approximately 30% of sporadic cases (Zurawel, R. H. et al. (2000) Genes Chromosomes Cancer 27(1):44-51; Kool, M. et al. (2008) PLoS ONE 3(8):e3088; Dellovade, T. et al. (2006) Annu. Rev. Neurosci. 29:539; Rubin, L. L. and F. J. de Sauvage (2006) Nat. Rev. Drug Discov. 5:1026). Mice heterozygous for Ptch1 (Ptch1+/−) can spontaneously develop medulloblastoma and treatment with Hh pathway inhibitors results in tumor elimination and prolonged survival (Goodrich, L. V. et al. (1997) Science 277(5329):1109-1113; Romer, J. T. et al. (2004) Cancer Cell 6(3):229-240). However, it has recently been observed that a patient treated with the novel Hh pathway inhibitor, GDC-0449 initially showed a dramatic response to treatment (Charles M. Rudin et al. (2009) N. Engl. J. Med. (submitted)), only to fail to have a durable response to treatment and a relapse of the tumor.
BCC is the most common human cancer and is predominantly driven by hyperactivation of the Hh pathway (Oro et al., 1997; Xie et al., 1998). The association between Hh signaling and cancer was first discovered in patients with Gorlin or basal cell nevus syndrome (BCNS), who are highly susceptible to medulloblastoma (MB) and BCC. These patients generally possess heterozygous germline mutations in Patched 1 (PTCH1), which encodes a receptor for Hh ligands (Hahn et al., 1996; Johnson et al., 1996). Hh ligand binding relieves PTCH1 suppression of the serpentine transmembrane (TM) signal transducer Smoothened (SMO). The vast majority of sporadic BCCs are driven by inactivating mutations and loss of heterozygosity (LOH) in PTCH1, with most of the remainder harboring activating mutations in SMO (Reifenberger et al., 2005). SMO promotes the activation and nuclear localization of GLI transcription factors by inhibition of Suppressor of fused (SUFU) and Protein kinase A (PKA). SUFU negatively regulates the Hh pathway by binding and sequestering GLI transcription factors in the cytoplasm (Stone et al., 1999). Loss-of-function mutations in SUFU are also associated with Gorlin Syndrome (Pastorino et al., 2009; Smith et al., 2014; Taylor et al., 2002). Approximately 50% of sporadic BCCs also have TP53 mutations (Jayaraman et al., 2014).
Several Hh pathway inhibitors (HPIs) are currently under clinical investigation for both BCC and MB (Amakye et al., 2013). Vismodegib, previously known as GDC-0449, is a SMO inhibitor approved for the treatment of metastatic and locally advanced BCC (Sekulic et al., 2012). The majority of BCC patients treated with vismodegib experience a clinical benefit, including both complete and partial responses (Sekulic et al., 2012).
However, a preliminary estimate suggests that up to 20% of advanced BCC patients develop resistance to vismodegib within the first year of treatment (Chang and Oro, 2012). To date, the only functionally characterized mechanism of acquired resistance to vismodegib in the clinic came from a patient with metastatic MB. A SMO-D473H mutation was detected in a biopsy from a relapsed metastatic tumor and was shown to abrogate drug binding in vitro (Yauch et al., 2009). Four other clinical SMO mutations were recently reported in vismodegib-resistant BCC, but were not examined functionally (Brinkhuizen et al., 2014; Pricl et al., 2014). Several resistance mechanisms to SMO inhibitors have been delineated from preclinical models, including additional SMO mutations, amplification of downstream Hh pathway components such as GLI2, and activation of bypass signaling pathways including phosphatidylinositol 3-kinase (PI3K) kinase and atypical protein kinase C ι/λ (aPKC-ι/λ) (Atwood et al., 2013; Buonamici et al., 2010; Dijkgraaf et al., 2011). However, it remains unclear which mechanisms drive resistance in patients.
There is an urgent need in the art to identify additional GDC-0449-resistant mutant SMO proteins and to find compounds that modulate SMO activity in such mutant SMO proteins to overcome drug resistance upon treatment with GDC-0449. There is further a need to a method to diagnose patients who may be resistant to treatment either through natural variation of their SMO genotype or through acquired mutation and resistance.